Ceramic articles prepared from metal nitride powders are of great interest because of their excellent thermal, electrical and mechanical properties. For example, AlN is useful as a ceramic substrate due to its high thermal conductivity, high electrical resistivity, and thermal expansion match with silicon. To prepare a ceramic article, such as a substrate, it is necessary to consolidate the powder into the desired shape and sinter (fire or heat treat) to a dense body. In order to form a dense article it is necessary to start with a powder of fine particle size and a high specific surface area. To achieve the desired thermal conductivity, electrical resistivity and mechanical properties, it is vital that the powder be pure and be able to sinter to theoretical density. Therefore, in order to produce a useful ceramic article of a metal nitride possessing the desired properties, it is necessary to begin with very fine, high surface area, high purity powders.
For the commercial production of aluminum nitride powder two methods are in common use. The direct nitriding method involves exposing metallic aluminum to a nitrogen or ammonia atmosphere at elevated temperatures. Pulverizing of both the original aluminum and the resulting aluminum nitride is required, with the disadvantages of introducing impurities and causing partial oxidation of the nitride. Moreover, unreacted aluminum remains as an impurity, degrading the properties of the AlN. In the second method, i.e., carbothermic reduction, alumina and carbon powders are intimately mixed and heated under nitrogen or ammonia, followed by a second heating step in air to remove residual carbon. Residual alumina remains as an impurity, and milling is usually required to reduce the aluminum nitride particles to a size suitable for fabrication by sintering. Such processes also require high reaction temperature and lengthy reaction times.
A carbothermic process reportedly providing aluminum nitride of somewhat improved purity and small particle size is described by Kuramoto et al., U.S. Pat. No. 4,618,592. Intimate mixing of fine powders of alumina and carbon of high purity in a liquid medium are required before the firing steps. Another carbothermic process is described by Mitomo et al., U.S. Pat. No. 4,643,859. An alkoxide of aluminum is first formed and blended with carbon particles in a suitable solvent. The alkoxide is then hydrolyzed, the solvents removed, and the resulting powder calcined under nitrogen.
Micheli, U.S. Pat. No. 4,627,966, describes the production of sinterable metal-oxygen composition powders. An aqueous solution containing multivalent metal cations is mixed with an aqueous solution of ammonium polyacrylate. The precipitate of metal polyacrylate is separated, the organic portion burned out and the ash calcined in air to produce the desired metal oxide. Holler, U.S. Pat. No. 3,908,002, describes the production of high surface area alpha alumina. A solution in a non-aqueous solvent of an aluminum salt is mixed with a non-aqueous solution of a polycarboxylic acid. The resulting precipitate is thermally decarboxylated in vacuo, heated at above 1000.degree. C. in an inert, dry atmosphere to provide a phase conversion to alpha alumina, and finally heated in an oxygen-containing atmosphere to remove the residual carbon.
The patent to Mitomo et al., U.S. Pat. No. 4,643,859, also points out that known methods for producing aluminum nitride have been: (a) direct nitriding by heating aluminum metal in nitrogen, (b) reduction - nitriding by heating a mixture of alumina and carbon in nitrogen, and (c) reaction of an aluminum compound in the gas phase with nitrogen or ammonia. Each method has disadvantages. Method (a) requires a catalyst and the resulting nitride is impure. In method (b) it is difficult to mix uniformly the starting materials and problems exist with the requirement for excess carbon. Method (c) is expensive and requires large scale apparatus.
All of the prior methods of forming metal nitrides have had drawbacks, including the method of Mitomo et al., which, like method (b) above, uses elemental carbon. It is impossible or exceedingly difficult to obtain a homogeneous mixture of the aluminum species and the carbon species when elemental carbon powder is used.
A distinguishing feature of the applicants' method is that elemental carbon is not used. Instead, a water dispersible or soluble carbon compound is used which mixes intimately and homogeneously with the aluminum compound, in effect, coating the colloidal particles of the aluminum compound. As a result, the nitride powders are of great purity and extremely small particle size. The method and its product differ markedly from the prior art.